A gas engine emits blue smoke for a few seconds after starting cold, the most likely cause is

• A. Leaking head gasket
• B. Worn valve seals
• C. Overheating
• D. Faulty oxygen sensor

Answer: (B)

Blue smoke indicates oil is burning in the combustion chamber. When an engine is started cold, worn valve stem seals can allow small amounts of oil to seep past into the cylinder during valve opening. This oil burns off as the engine warms, so the blue smoke appears for a short time after starting and then disappears. The seals are a common source of startup smoke because they directly control oil entering the combustion chamber at low engine temperatures.

Leaking head gaskets typically cause coolant loss and white or greenish smoke from steam, not a brief blue haze at startup. Overheating often produces steam and white smoke rather than a short-lived blue smoke. A faulty oxygen sensor affects fuel mixture and exhaust emissions, not oil burning, so it wouldn’t cause blue smoke at startup.